The invention relates to a sheet transport system, particularly for transporting documents of value such as banknotes.
Banknote transport systems are used to transport banknotes for a variety of purposes such as dispensing and accepting and also sorting in which banknotes are sorted in accordance with their denomination or authenticity. Typically, such sorters are provided with a number of transport sections for conveying banknotes through the system, detectors being provided along the banknote path to detect certain characteristics of the banknotes, these characteristics being used to actuate diverters to divert the banknotes to suitable storage locations. Typically, all the transport sections are powered by a single AC induction motor, the motive power from this motor being distributed to drive points known as capstans. The capstans in turn drive transport belts for transporting the banknotes.
Recently, there has been a tendency to enlarge the size of the machine, increasing the number of capstans required, to increase the speed of the transport system in order to improve the rate at which notes are processed.
The power from the single AC motor is transmitted to the capstans using a system of toothed belts and a drawback of this approach is that these belts consume a large percentage of the motor""s power, generating considerable levels of noise and vibration. They also require a machine frame of considerable strength and occupy a large volume of space within the machine and inhibit transport arrangement design.
In accordance with the present invention, a sheet transport system comprises a plurality of separately driven transport sections, each transport section being driven by a respective motor or motors; a control system for controlling operation of the motors; and a monitoring system for monitoring the performance of the motors, the control system being responsive to the monitoring system to maintain correct operation of the motors.
In this invention, we propose a distributed drive approach in which each transport section is driven by its own motor or motors. This has a number of significant advantages. Firstly, a significant power saving is obtained. For example, a machine which uses 3 kW power based on a single AC induction motor can be run instead using a distributed drive system with a total motor output power of 1 kW. Further advantages include greatly reduced noise output, a light machine build, more space available within the system, and reduced manufacturing cost.
Each transport section is typically defined by a pair of friction feed belts, at least one of the friction feed belt being entrained around at least one drive capstan, directly driven by a respective motor. However, other forms of flexible transport conveyor mechanism are also envisaged.
In this context, we avoid the use of toothed timing belts and the like to connect each motor to the respective drive capstan and the term xe2x80x9cdirectlyxe2x80x9d should be construed accordingly.
The motor could be coupled with the drive capstan via a gear system but preferably the drive capstan is mounted to a drive shaft, a rotor of the corresponding motor being directly secured to the drive shaft. Most conveniently, the drive shaft is journalled between spaced sets of bearings, the motor being located between the sets of bearings.
Thus, in the most preferred arrangement, a brushless DC servomotor could be used in place of the capstan bearing housing to achieve long life and high efficiency. The servomotor would form a bearing housing in its own right. Since the need to support high side loads from toothed timing belts will have been removed, smaller motor bearings will be capable of supporting the capstan.
A DC motor is preferred because it is simple to drive and control. It has high torque at zero speed and is power efficient.
Preferably, the control system comprises a plurality of digital controllers, one connected to each of the motors. The use of digital controllers is particularly preferred since these can provide a very accurate control of the motors. To that end, preferably, the motors behave in the manner of stepper motors, the digital controllers generating a control signal which defines a number of steps to be performed by the motor. In this case, the digital controllers act as position controllers to obtain a very precise movement in response to a demand or control signal. The advantage of using a position controller as a rate controller (to control the speed of the motor) is that the rate of rotation will be exactly the same as the rate at which the demand pulses are supplied. When combined with the digital control method, the resulting controller can cause a motor to run at a speed exactly related to the demand pulse frequency with no analogue settings or adjustments and no build up of error in position with time.
It is important to monitor the performance of each motor to ensure that it is being driven correctly. For this reason, the monitoring system is provided which could be used to monitor movement of the motor or preferably is adapted to monitor rotation of the drive capstans.
Conveniently, each controller includes a comparator for comparing the required performance of the corresponding motor with its actual performance, as reported by the monitoring system, and for modifying a control signal supplied to the motor to compensate for any difference. This provides a simple feedback system to ensure that the motor responds correctly to a demand or control signal.
One problem which can arise in sheet transport systems in which some motors are mechanically linked, via engaging friction feed belts of different transport sections, is that the friction between the belts may vary from a large value when the belts are new to a low value as they age. Furthermore, tolerances in belt thickness, capstan diameter and the type of belt paths can cause motor shafts to run at slightly different speeds and this variation can cause a variation in the way in which each motor responds to a demand or control signal. Although each motor could be driven individually by the control system, it is most convenient if the control system issues a single control signal to all the motors corresponding to the desired speed. In that case, preferably each controller includes an electronic gear box for adjusting the response of a motor to an input control signal, the control system including means for altering the adjustment provided by the electronic gear box so as to balance the performance of mechanically linked motors.
The use of an electronic gearbox allows the response of each motor to be adjusted with respect to the other motors and mechanically linked to it to compensate for these problems.
It should be understood that in a typical sheet transport system, not all the transport sections will be separately driven and some could be driven by frictional engagement with driven transport sections.
The transport sections also need not necessarily only include friction feed belts but could include other components of a sheet transport system such as sheet stackers, shredders and the like.